Topographical distribution of delay-tuned responses in the mustached bat inferior colliculus

In the mustached bat, delay-tuned neurons respond best to specific delays between the first harmonic frequency modulated (FM) component (FM1; 24-29 kHz) of the emitted biosonar pulse and a higher harmonic FM component in returning echoes (e.g. FM3, 72-89 kHz). These delay-tuned, combinatorial responses predominate in the inferior colliculus (IC) of the mustached bat. This study examined the topographical distribution of delay-tuned neurons in the 72-89 kHz frequency representation of the IC. We recorded and histologically localized 163 single units. Ninety units were facilitated and 41 were inhibited by the combination of two frequencies in the 24-29 kHz and 72-89 kHz ranges. The facilitatory responses were selective for delays up to 20 ms between the two signals. To determine if delay-tuned neurons were topographically organized, we plotted the dorsomedio-ventrolateral and caudo-rostral positions of each unit versus its best delay. Best delay was not correlated with either location. Response latency to best frequency tones was topographically organized, but was not correlated with best delay. This indicates that the latency axis in the IC is unrelated to the delay tuning of these combinatorial neurons. Because delay-tuned neurons are not topographically organized in the IC but are in the auditory cortex, our findings suggest that the creation and organization of delay-tuned neurons occur at different stages in the ascending auditory system.